Japanese Patent No. 2509103 discloses a technology of inserting a projection bolt held by a holding head of an advancing-retracting feed rod into a receiving hole provided in an electric resistance welding electrode, and then advancing the electrode to weld the projection bolt to the steel plate part on standby. Japanese Patent Application Laid-Open No. Hei. 9-57458 discloses a technology in which the action of moving a steel plate part clamped on a robot device is synchronized with the action of feeding a projection nut to an electrode. Japanese Patent Application Laid-Open No. 2003-236672 discloses a technology of advancing an electrode in response to a signal obtained when a feed rod is located at a predetermined position on its return stroke and then carrying out pressurization and energization.    [Patent Document 1] Japanese Patent No. 2509103.    [Patent Document 2] Japanese Patent Application Laid-Open No. Hei. 09-57458.    [Patent Document 3] Japanese Patent Application Laid-Open No. 2003-236672.